1. Field of Invention
This invention pertains to a low phonon energy glass and glass fiber doped with a rare earth element.
2. Description of Prior Art
Low phonon energy glasses, which includes fluoride, sulfide, selenide and telluride glasses, have become extremely important as hosts for rare earth ions. For these systems doped with rare earths, radiative emissions normally quenched in oxide hosts have improved efficiencies. An example is the .sup.1 G.sub.4 -.sup.3 H.sub.5 1.3 .mu.m transition for praseodymium in a fluoride host. This transition in the fluoride glass has been developed and commercialized as a fiber amplifier. This system shows poor gain since the transition has a low quantum efficiency due to the high multiphonon relaxation rate for the level in the fluoride host. This has motivated research into development of glasses with even lower phonon energies than the fluoride glass hosts.
Sulfide glass hosts have emerged as potential candidates for active applications at the 1.3 .mu.m wavelength. Although the optical efficiency for praseodymium is higher than in the fluoride host, there are a couple of factors which limit the usefulness of the sulfide glass host. Firstly, the stability of the sulfide glasses used to-date is low and leads to crystallization during fiber drawing. Secondly, the rare earth ions tend to decrease the stability further which leads to more crystallization problems. As the absorption cross section for praseodymium is low, long fiber lengths are required for efficient device operation. Low loss fiber of about 0.1 dB/m or less is required, and to date, fabrication of low-loss sulfide fibers doped with a rare earth has not been realized due to significant crystallization during the fiber drawing. Therefore, there is a need for stable, rare earth doped, low phonon energy chalcogenide glasses which can be drawn into low loss fibers having a loss of less than 1 dB/m.
There is a need for bright sources in the infrared for infrared scene simulation and characterization of focal plane arrays on the Joint Strike Fighter and other seeker platforms. Furthermore, these bright fiber sources can be used in the fiber optic chemical sensor applications for facility clean up and in other government and industrial applications.
A companion application entitled "Infrared Transparent Selenide Glasses" was filed in USPTO on Mar. 14, 1997, and bears Ser. No. 08/818,204 discloses a glass comprising an alkaline earth selenide modifier, is now issued as U.S. Pat. No. 5,846,889, germanium selenide, gallium selenide and/or indium selenide, optionally doped with a rare earth. The glass components can be used as elements. The modifier is barium selenide or strontium selenide. Fibers from this glass have measured loss of greater than 10 dB/m whereas fibers made from the glass described herein have loss of less than about 1 dB/m.